• Question: How do scientists know how planets there are, it is not like they have been to them or gone by them and how do they know that Pluto is the smallest planet? They can't be sure, for the further away you are from the sun, the colder you get.

    Asked by pattymbaji to Ben, Dave, Ed, Sam, Susana on 18 Jun 2013.
    • Photo: Benjamin Hall

      Benjamin Hall answered on 18 Jun 2013:

      You’re absolutely right. We don’t know!

      We only know about the planets we’ve observed through a telescope and can see. It’s entirely possible (but very unlikely) that we’ve missed some planets in our solar system.

      What we can be sure of is that there are far more planets in the universe than the ones we can see in our solar system!

      Also, I’m not sure Pluto is classified as a planet any more…

    • Photo: David Briggs

      David Briggs answered on 18 Jun 2013:

      There are 8 planets in the solar system. We know this through just looking with telescopes! We have sent space probes to all 8 of the planets. Pluto has been re-named as a dwarf planet. There are lots and lots of dwarf planets in the solar system. But you are right, we have almost certainly not seen all the dwarf planets around our sun.

      As for outside our solar system, we have detected more that 700 planets orbiting other stars – we can see the effects that they have whilst they orbit their stars. Some of them are predicted to be the right temperature for liquid water (and maybe life!) to exist.

    • Photo: Sam Horrell

      Sam Horrell answered on 19 Jun 2013:

      It’s true, pluto has been downgraded form from a planet to a dwarf planet. Poor pluto.

      Calculating the size of a planet comes down to the planet’s radius, the planet’s gravity and the distance from the sun or another star. Using some clever trigonometry and other maths you can plug these values into a formula and it gives you the mass.

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